Lasers are sometimes operated as wavelength-sweeping devices in remote sensing, LIDAR, and optical coherence tomography applications, as well as to test telecommunications components, among other applications. Discontinuities in the wavelength sweep or operation of the wavelength outside of a single mode can significantly affect the application that the laser is being used in. For example, the shape of a molecular gas absorption feature may be distorted by a discontinuity—a forward or backward jump—or operation of the wavelength outside of a single mode in the wavelength sweep of the laser. In another example, wavelength discontinuities or operation of the wavelength outside of a single mode can reduce the signal-to-noise of an OCT measurement of tissue, or a LIDAR (Laser RADAR) measurement using the Frequency Modulated Continuous Wave (FMCW) technique or more complex lidar/radar waveforms. Thus, it is usually desirable to eliminate from swept-wavelength lasers wavelength discontinuities, wavelength non-linearity, and operation of the laser outside of a single mode.
Attempts in the prior art to maintain single mode operation and to control a sweep profile for a laser are numerous, but unsatisfactory. Although it may be possible to carefully maintain single mode operation and control a sweep profile using a calibrated parameter set at an initial point in time (often referred to as pre-emphasis or predistortion), the passage of time or changes in, e.g., temperature or humidity will create discontinuities, non-linearities, and cause operation outside of a single mode. For example, mechanically-tuned external cavity lasers operate in near continuous single-mode using an external cavity mechanism coupled with a gain medium. Another example is the use of pre-emphasis to control a semiconductor laser.
In a typical single mode tunable laser, there are two key elements; a method for changing the wavelength (either internal to the cavity, or external), and another for altering the cavity length to optimize side mode suppression and to maintain single mode operation. In an analogous tunable laser, known as Littman-Metcalf configuration, there is a specific mechanical configuration which constrains the change in wavelength to happen coincident with a commensurate change in cavity length, thus maintaining single mode operation. In these mechanical systems, there is a mechanical construction which constrains the mechanical “path” which is traversed to one in which the wavelength increase is linear, and the path length difference is simultaneously changed in concert with the wavelength increase to maintain single mode operation with good SMSR. Further, in a laser that uses external feedback, the current through the gain medium is another parameter to optimize the SMSR or the wavelength tuning, making such lasers tunable via a multivariate parameter space.
Operation of the mechanical laser is maintained through accurate, tightly-toleranced components and precision alignment of the cavity, or using real-time elements such as piezoelectric transducers that adjust the cavity length in real-time. Other laser configurations use an intra-cavity element. Over time, however, the alignment of the laser degrades or the components wear, which may cause changes in the sweep profile versus time and operation outside of a single mode. As the ambient temperature, humidity, or pressure change, the alignment can degrade, which can also cause changes in the sweep profile versus time and operation outside of a single mode. Vibrations external or internal to the laser may also misalign the cavity, which again may cause changes in the sweep profile versus time and operation outside of a single mode.
Even in lasers with stable cavities, it is difficult to create wavelength sweeps (for example, using pre-emphasis) without wavelength discontinuities. Monolithically-constructed semiconductor lasers, or non-semiconductor monolithic lasers in general, are a class of single-mode laser for producing swept wavelengths. Monolithic semiconductor lasers include several sections or segments in the semiconductor, which serve, for example, as adjustable cavity mirrors, laser gain, coupled cavities, cavity phase and (optionally) external amplification. Examples are Vertical Cavity Surface Emitting Lasers (VCSELs), VCSELs with Micro-electromechanical systems (MEMS) tuning structures, Vernier-tuned Distributed Bragg Reflector (VT-DBR) lasers, Vernier-tuned ring lasers, Y-branch lasers, coupled cavity lasers, discrete mode lasers, injection-locked or externally-stabilized lasers, Super-Structure Grating Distributed Bragg Reflector (SSGDBR) lasers and similar devices. Because these lasers are typically monolithic with no moving parts (excepting the MEMs devices), their cavities and associated optics are extremely stable and can operate in single-longitudinal mode with narrow linewidth and long coherence length. Tunable monolithic lasers of this class require multiple control signals to tune the wavelength, presenting a challenge to creating wavelength sweeps without wavelength discontinuities.